A liquid transfer article, such as a roll, is used in the printing industry to transfer a specified amount of a liquid, such as ink or other substances, from the liquid transfer article to another surface. The liquid transfer article generally comprises a surface with a pattern of depressions or wells adapted for receiving a liquid and in which said pattern is transferred to another surface when contacted by the liquid transfer article. When the liquid is ink and the ink is applied to the article, the wells are filled with the ink while the remaining surface of the article is wiped off. Since the ink is contained only in the pattern defined by the wells, it is this pattern that is transferred to another surface.
In commercial practice, a wiper or doctor blade is used to remove any excess liquid from the surface of the liquid transfer article. If the surface of the coated article is too coarse, excessive liquid, such as ink, will not be removed from the land area surface of the coarse article thereby resulting in the transfer of too much ink onto the receiving surface and/or onto the wrong place. Therefore, the surface of the liquid transfer article should be finished and the wells or depressions clearly defined so that they can accept the liquid.
A gravure-type roll is commonly used as a liquid transfer roll. A gravure-type roll is also referred to as an applicator or pattern roll. A gravure roll is produced by cutting or engraving various sizes of wells into portions of the roll surface. These wells are filled with liquid and then the liquid is transferred to the receiving surface. The diameter and depth of the wells may be varied to control the volume of liquid transfer. It is the location of the wells that provides a pattern of the liquid to be transferred to the receiving surface while the land area defining the wells does not contain any liquid and therefore cannot transfer any liquid. The land area is at a common surface level, such that when liquid is applied to the surface and the liquid fills or floods the wells, excess liquid can be removed from the land area by wiping across the roll surface with a doctor blade.
The depth and size of each well determines the amount of liquid which is transferred to the receiving surface. By controlling the depth and size of the wells, and the location of the wells (pattern) on the surface, a precise control of the volume of liquid to be transferred and the location of the liquid to be transferred to a receiving surface can be achieved. In addition, the liquid may be transferred to a receiving surface in a predetermined pattern to a high degree of precision having different print densities by having various depth and/or size of wells.
Typically, a gravure roll is a metal roll with an outer layer of copper. Generally, the engraving techniques employed to engrave the copper are mechanical processes, e.g., using a diamond stylus to dig the well pattern, or photochemical processes that chemically etch the well pattern.
After completion of the engraving, the copper surface is usually plated with chrome. This last step is required to improve the wear life of the engraved copper surface of the roll. Without the chrome plating, the roll wears quickly, and is more easily corroded by the inks used in the printing. For this reason, without the chrome plating, the copper roll has an unacceptably low life.
However, even with chrome plating, the life of the roll is often unacceptably short. This is due to the abrasive nature of the fluids and the scrapping action caused by the doctor blade. In many applications, the rapid wear of the roll is compensated by providing an oversized roll with wells having oversized depths. However, this roll has the disadvantage of higher liquid transfer when the roll is new. In addition, as the roll wears, the volume of liquid transferred to a receiving surface rapidly decreases thereby causing quality control problems. The rapid wear of the chromeplated copper roll also results in considerable downtime and maintenance costs.
Ceramic coatings have been used for many years on anilox rolls to give extremely long life. Anilox rolls are liquid transfer rolls which transfer a uniform liquid volume over the entire working surface of the rolls. Engraving of ceramic coated rolls cannot be done with conventional engraving methods used for engraving copper rolls; so these rolls must be engraved with a high energy beam, such as a laser or an electron beam. Laser engraving results in the formation of wells with a new recast surface about each well and above the original surface of the roll, such recast surface having an appearance of a miniature volcano crater about each well. This is caused by solidification of the molten material thrown from the surface when struck by the high energy beam. Thus the recast surface should be removed for most printing applications.
In offset printing the printing plate is not directly applied to the paper but first transfers its image to an offset blanket cylinder which is a flexographic surface, such as rubber, and the image is transferred from the blanket cylinder to the paper. Printing ink is applied to the plate cylinder by ink transfer or ink metering rolls which may be a single roll or may be a series of rolls. In lithography, the image and non-image areas are on the same plane on the printing plate but the image area is grease receptive and water-repellent whereas the non-image area is water-receptive and grease-repellent. The ink solvent therefore adheres only to the image areas, from which it is transferred to the surface to be printed, usually by the offset method.
In the printing cycle of lithography, water or "fountain solution" is fed to the printing plate roll just before it contacts the ink transfer rolls. This is usually done by means of rollers which meter the amount of water applied. The moisture film produced on the printing plate roll is continuous on the non-image areas of the plate and acts as a barrier preventing adhesion of ink. Any moisture on the greasy image areas is discontinuous and does not prevent transfer of ink to them.
For lithographic ink, careful selection of ingredients is essential. Since the ink comes into intimate and continuous contact with water during printing, it must be free from any tendency to bleed or to form an ink-in-water emulsion. The formation of water-in-ink emulsion is unavoidable, but this does no harm unless the working consistency of the ink is damaged. During normal printing, the ink takes up from 5% to 30% of water as a water-in-ink emulsion. There is, however, still very little known about the surface chemistry of this ink/water relationship.
The ink transfer or metering roll surface must be oleophilic so that it receives the greasy printing ink into the wells engraved on its surface and must also be hydrophobic so that it repels water which is on the surface of the printing roll. Traditionally, an ink transfer roll has been made with a pore-free surface of copper which has been found to be both oleophilic and hydrophobic. In practice and as stated above, copper surfaces are relatively soft and are not hard wearing, and therefore the surface of the engraved copper roll generally has been coated with a layer of pore-free chromium to increase its resistance to wear. Such wear is particularly evident where doctor blades are used to meter the amount of ink transferred. The application of a doctor blade however does produce continuous wear on the surface of the transfer roll and much consideration has been given in the past to the production of ink transfer or ink metering rolls which are much more resistant to wear.
First an attempt was made to coat the copper with a microporous layer of ceramic. It presumably was thought that the microporous ceramic would retain the oleophilic and hydrophobic properties of the pore-free copper surface while improving its wear-resistant properties. In practice this was found not to work and satisfactory transfer of ink evenly over the printing areas of the plotting plate was not achieved in practice.
Another attempt to solve the problem of wear was to coat the base roll with a ceramic and engrave the surface of the ceramic. Of these base rolls, those coated with a chromium oxide layer and engraved with a pattern of wells by a pulsed laser beam technique did solve the problem of wear. However, another problem was found to arise in the case of such ceramic coated rolls in that after a while it was found that the transfer of ink became patchy. The reason was found to be that some of the wells in the ink well pattern engraved on the ceramic surface changed their properties from oleophilic to oleophobic thus reducing the amount of ink transferred over areas of the transfer roll where this occurred thereby forming an uneven application of ink to the printing roll.
To avoid this, such rolls have been coated with a thin pore free layer of copper which, of course, is known to have the surface properties needed for ink transfer over long periods without this disadvantage occurring.
German Patent Application DE No. 3713027A1 discloses a liquid transfer roll having a multiplicity of wells and wherein the wells contain a moisture-repellant coating of a material such as vapor-deposited copper, nickel, silicon, asphalt or a suitable synthetic in the form of Teflon which is a trademark of E. I. DuPont de Nemours & Co. for polytetrafluoroethylene or Mylar which is a trademark of E. I. DuPont de Nemours & Co. for polyester film made from polyethylene terephthalate resin.
It has now been found that very thin films of certain vapor-deposited polymers when applied to the surface of the wells in a ceramic coated liquid transfer roll will fill any microporosities in the surface caused by the laser engraving and prevent the change in surface characteristics of the wells from oleophilic to oleophobic and from hydrophobic to hydrophilic.
In its broadest aspect therefore, the present invention provides a liquid transfer article, such as an ink transfer roll for use in offset printing, which comprises a base roll coated with a ceramic layer and having engraved in said ceramic layer a pattern of liquid receiving wells, characterized in that the microporosities in the surface of each of said wells is filled with a film of a vapor-deposited polymer.
The invention also provides a method of forming a liquid transfer article for use in offset printing comprising coating a base substrate with a layer of ceramic or metallic carbide and engraving a pattern of liquid receiving wells on the surface of the coating by means of a laser beam and then vapor depositing a polymer to fill any microporosities in the surface of the wells.
It is an object of the present invention to provide a thin film of a vapor-deposited polymer on the surface of wells engraved in a liquid transfer article.
It is another object of this invention to provide a liquid transfer article having a pattern of engraved wells in which any microporosities in the surface of the wells are filled with a vapor-deposited polymer.
It is another object of the present invention to provide a liquid transfer roll for use in the printing industry with a ceramic coating having a pattern of engraved wells coated with a thin film of parylene which effectively fills any microporosity in the surface of the wells.
It is another object of the present invention to provide a method for forming a liquid transfer article having a pattern of engraved wells with a vapor-deposited polymer filling any microposorities in the surface of the wells and coating the surface of the wells with a thin layer of the polymer that exhibits oleophilic and hydrophobic characteristics.
The above and further objects and advantages will become apparent upon consideration of the following description thereof.